Portable equipment such as a cell phone or PDA (personal digital assistant), by its nature, requires a thin, light display device. Frequently used display device therefor is a liquid crystal display device or an organic light emitting diode display device (hereinafter referred to as ‘OLED display device’). In general, a liquid crystal display device realizes display incorporating a liquid crystal display panel forming an image by controlling transmittance or reflectance of light, and a backlight device and a front light device as a light source. In addition, the OLED display device can realize display without incorporating a separate light source because the OLED display panel itself emits an image light.
When the display device is seen in a bright environment, the surface of the screen reflects ambient light so that an image on the screen is sometimes not easily visible. Because of this, the screen nowadays has an antireflection layer to control the reflection of ambient light.
Multi-layer films as well as single-layer films are used as antireflection layers. The antireflection layer with a multi-layer structure is realized by alternately layering films having a high refractive index and films having a low refractive index by deposition methods such as evaporation, sputtering or ion plating. Although excellent reflection prevention is achieved, its manufacturing cost is high.
On the other hand, the single-layer film is formed of a single coating layer and thus a reduced number of processes are required, leading to a low-cost manufacturing process. However, improvement of antireflection performance is still required.
In case of the single-layer film, its refractive index R′ can be expressed by the formula below, in which n0 denotes refractive index of an outside medium, n1 denotes refractive index of the surface of a display device, and n2 denotes refractive index of the antireflection layer.R′={(n22−n0×n1)/(n22+n0×n1)}2  (1)
Here, the outside medium is typically air and its refractive index, n0, is usually 1.0. Thus, the refractive index R′ can be expressed as:R′={(n22−n1)/(n22+n1)}2  (2)
When n2=√n1, theoretically, reflectance becomes 0%.
A proper refractive index of the antireflection layer is about 1.22 because the refractive index of the surface of a display is typically 1.5. At present state, however, a fluorine-series resin known to have a low refractive index has the refractive index of 1.34, so it is very difficult to get a sufficient antireflection performance from the single layer antireflection layer containing a fluorine-series resin.
In recent years, methods for lowering refractive index of a single-layer film have been suggested and a thin film disclosed in Japanese Patent Laid-Open No. 2003-201443 is one of them. This thin film essentially comprises microparticles having an internal cavity (hollow microparticles) and a binder for protecting and retaining the hollow microparticles. Because the internal cavity of the thin film has substantially the same refractive index with air (nair=1.0), even though the material of the hollow microparticles or the binder for protecting and retaining the hollow microparticles may have a large refractive index, refractive index from the viewpoint of the film is very close to the refractive index of air. That is to say, by forming a substrate with this film, reflectance of the device can be lowered.
Another example of such methods for lowering the refractive index of a single-layer film is by employing a film with a low refractive index as disclosed in Japanese Patent Laid-Open No. H7-92305. According to this technique, the surfaces of the organic ultrafine microparticles on the side near to air are exposed and a rugged film is formed on the surface, lowering density on the surface, and resultantly forming a low refractive index film.
Still another example of such methods is employing a low refractive index film having honeycomb structured pores as disclosed in Japanese Patent Laid-Open No. 2004-83307. According to this patent document, a plurality of honeycomb-structured pores capable of penetrating silica particles are formed in parallel to each other. As a result, a maximum void ratio can be obtained without deteriorating strength of the silica particle itself. Therefore, a low refractive index film having superior mechanical strength can be manufactured.